Handle case when stage contains colons

When I refactored Mercury's deployment script to allow for multiple projects, I did so by adding colons in stage names.  For example:

```bundle exec cap mercury:staging deploy:build```

The ```mercury:staging``` bit is the stage name.  Unfortunately this caused problems when creating related Git tags.  So, these colons are converted to dashes.

This has been in use since May, but please let me know if there are any concerns.